The universe is full of stars. So why doesn’t the light from all of them add up to make the whole sky bright all the time?
We know that every direction we look into space we would be looking at a star. Yet the space is black. This is known as olber’s paradox.
When the paradox was first proposed by Olber, it was believed that the universe was static and infinitely old.
But this static universe theory could not explain the reason of the paradox.
According to the big bang theory we know that the universe is expanding and it is 15 billion years old.
As the universe is expanding, the light from distant stars travelling towards us is also expanding.
Due to expansion the wavelength of the light increases and gets shifted to the wavelength that is not visible to human eyes.
And also as the universe is only 15 billion years old, we are confined to see the light that is 15 billion light-years away.
The light from the stars farther away have not yet reached us and do not contribute to the light in space.
Hence the space is black.
Paradox Pegasus 